1. Field of the Invention
Bodies of heat-resistant aluminum alloys which are produced from powders with high cooling rate obtained by atomizing a melt. High content of alloy constituents which are not permissible under otherwise standard solidification conditions such as, for example Fe, Cr and V.
The invention relates to the production of moldings with improved mechanical properties starting from aluminum alloys.
In particular, it relates to a method for producing a heat-resistant aluminum-alloy workpiece having high transverse ductility which is manufactured from a compact produced by powder metallurgy, in which alloy powder of the final composition or a mixture of prealloy powders is first cold-isostatically pressed under a pressure of 1500 to 5000 bar and the extrusion billet produced in this manner is recompacted in the chamber of an extrusion press by hot pressing and is extruded immediately afterwards to form a compact, and piece is cut off the compact for further shaping.
2. Discussion of Background
The following literature is cited in relation to the prior art: "High-strength powder metallurgy aluminium alloys", edited by M. J. Koczak and G. J. Hildeman, TMS-AIME, 1982, pages 63-86: M. Rafalin, A. Lawley and M. J. Koczak, "Fatigue of high-strength powder metallurgy aluminium alloys".
In the document mentioned attention should be paid, in particular, to FIG. 1.
The production of workpieces-powder metallurgy manufacture is normally carried out by upsetting a compact or a bar section in the direction of the main axis (usually the axis of rotation) and subsequent forging. Compare also FIGS. 1 to 4 in this document.
FIG. 1 shows a perspective representation of a compacting process. The aluminum-alloy powder is compacted in a press to form a compact body 1. The externally applied compressive forces are indicated by arrows. Usually such bodies 1 are produced by hot pressing and have, as a rule, a cylindrical shape. A first step in the method may, however, also be cold pressing or cold isostatic compacting (not shown).
FIG. 2 relates to a perspective representation of an extrusion process. The compressive forces acting from the outside are again indicated by arrows which coincide with the extrusion direction and the longitudinal axis of the body. 2 is the already partially extruded extrusion billet having the normal cylindrical shape. 3 is the extruded bar resulting therefrom and having, as a rule, a circular cross-section. 4 represents a cylindrical bar section.
FIG. 3 shows a perspective representation of an upsetting process. The elongated cylindrical bar section 4 shown by broken lines is deformed by axial compressive forces (indicated by arrows) to form a forged cylindrical blank 5 in the form of a flat disk.
FIG. 4 relates to a perspective representation of a forging process. The blank 5 (FIG. 3) which is not shown is deformed by further steps in the method (compressive forces indicated by broken arrows) to form a die-forged finished body of revolution 6.
In this technique, the deformation takes place in all the steps in the method virtually uniaxially, i.e. in the direction of the original compressive forces in the first compacting (FIG. 1) or in the extrusion direction (FIG. 2). This has the result that the finished workpiece is strongly anisotropic and has strongly varying mechanical properties in the various directions. Highly heat-resistant alloys produced by powder metallurgy are, as a rule, difficult to deform. Owing to their low ductility at the comparatively low forging temperature, the mold filling capacity is poor and the crack susceptibility is high. If the extrusion process step is dispensed with, the deformation is inadequate. The ductility is very low in all directions. Although the ductility in the longitudinal direction (extrusion direction) meets the requirements if the extrusion step is introduced, it is very low at right angles to the extrusion direction. However, in bodies of revolution, the main load in operation is precisely in the plane which is perpendicular to the extrusion and upsetting direction. In addition, the ductility varies considerably from the core to the edge. The body behaves anisotropically, and this prevents its maximum exploitation in operation. Two examples may demonstrate this: